In recent years, attention has been focused on digital versatile discs (DVDs) as large-capacity optical recording media since digital information can be recorded at about six times as high a recording density as compared with compact discs (CDs). Particularly, in the case of reproducing/recording information on/from a high-density optical recording medium such as a DVD, it is necessary to shorten the wavelength of laser light and increase a numerical aperture (NA) of an objective lens in order to reduce a beam spot diameter of a laser beam. As a result, the optical recording medium comes to have a smaller margin for a tilt angle.
In order to increase a recording capacity, multilayer optical recording media each including a plurality of recording layers have been and are in widespread use. Here, reflectance may largely change between layers of a multilayer optical recording medium. Patent literature 1 discloses an optical disc player capable of stably performing a tilt servo even if a recording layer is changed during the reproduction from such an optical recording medium.
Here, an example of the above conventional optical disc player is described with reference to the drawing. FIG. 19 is a diagram showing the construction of a conventional optical disc player. In FIG. 19, identified by 171 is an optical disc, by 172 an optical head, by 173 a photodetector, by 174 a liquid crystal element, by 175 a motor, by 176 an Rf amplitude detector, by 177 a reproducing unit, by 178 a focus/tracking drive circuit, by 179 a liquid crystal drive circuit, by 1700 a focus servo unit, by 1701 a tracking servo unit, by 1702 a tilt servo unit and by 1703 a microcomputer.
The operation of the thus constructed optical disc player is described. The optical head 172 irradiates the optical disc with laser light, receives reflected light from the optical disc 171 and generates a signal corresponding to the amount of the received light. The optical disc 171 is rotated and driven by the motor 175. In the optical head 172, the liquid crystal element 174 for correcting an aberration in a disc radial direction is arranged on an optical axis of a light beam.
An output from the photodetector 173 is sent to the Rf amplitude detector 176, the focus servo unit 1700 and the tracking servo unit 1701. The focus servo unit 1700 feeds a control signal to the focus/tracking drive circuit 178 to focus a light beam emitted from the optical head 172 on a desired recording layer. Further, the tracking servo unit 1701 feeds a control signal to the focus/tracking drive circuit 178 so that a light beam emitted from the optical head is focused on a desired track on a desired recording layer. Here, the focus/tracking drive circuit 178 drives the optical head 172 to focus the light beam emitted from the optical head 172 on the desired track on the desired layer based on the above control signal.
The Rf amplitude detector 176 receives an Rf detection signal from the photodetector 173 and feeds the Rf amplitude signal to the tilt servo unit 1702. The tilt servo unit 1702 outputs a tilt drive signal to the liquid crystal drive circuit 179 using the Rf amplitude signal so as to maximize an envelop of the Rf amplitude signal, and the liquid crystal drive circuit 179 drives the liquid crystal element 174 based on this control signal. These controls are executed in accordance with a command from the microcomputer 1703.
Next, a time of an interlayer jump is thought. If an interlayer jump command is issued from the microcomputer 1703, the tilt servo unit 1702 retains and outputs a tilt drive value immediately before the jump. In other words, the tilt servo unit 1702 continues to output a constant value during the jump without depending on the intensity of the envelope during the jump. In this way, a situation of causing a reduction in a tilt control performance and losing the control can be avoided, wherefore a stable servo operation can be obtained.
Patent literatures 2, 3, 4 and 5 disclose the elaboration of a pulse or offset signal to be fed to a focus error signal to stably perform a focus control during the interlayer jump as described above.
However, in the optical disc player constructed as above, the interlayer jump in multilayer optical recording media having a higher density than DVDs become unstable. This is described in detail. First of all, an optical head is thought which records and reproduces information on and from a Blu-ray disc which is an optical recording medium having a higher density than DVDs. Since the optical head for recording and reproducing information on and from the Blu-ray disc includes a light source having a wavelength of about 405 nm and an objective lens having a very large aperture (about 0.85), base material thicknesses differ among the respective recording layers and the amounts of coma aberration largely differ even if an inclined amount is the same in the case of reproducing information from a multilayer optical recording medium including two or more recording layers using this optical head.
FIG. 20 is a construction diagram of a multilayer optical recording medium including three recording layers. As shown in FIG. 20, a three-layer optical disc 181 is composed of a base material 182, a first recording layer 183, a first intermediate layer 184, a second recording layer 185, a second intermediate layer 186, a third recording layer 187 and a protective layer 188 on the underside in this order from an optical head side 180. The base material 182 and the first and second intermediate layers 184, 186 are made of a transparent medium such as resin.
The first intermediate layer 184 is present between the first and second recording layers 183, 185 and the second intermediate layer 186 is present between the second and third recording layers 185, 187. Thus, the thickness from the surface of the optical disc 181 on the optical head side 180 to the second recording layer 185 is larger than the thickness from this surface to the first recording layer 183 only by the thickness of the intermediate layer 184, and the thickness of the surface of the optical disc 181 on the optical head side 180 to the third recording layer 187 is larger than the thickness from this surface to the second recording layer 185 only by the thickness of the intermediate layer 186.
Here, distances from the surface of the optical disc 181 on the optical head side 180 to the respective recording layers are called base material thicknesses of the respective recording layers. For the three-layer optical disc as shown in FIG. 20, there are cases where, when information is recorded or reproduced on or from the first recording layer 183 while a microspot of a light beam is focused on the first recording layer 183, the focus position of the microspot of the light beam is moved to the second recording layer 185 or conversely the focus position is moved from the second recording layer 185 to the first recording layer 183 to record or reproduce information on or from the second recording layer 185. An operation of moving the focus position to a different recording layer in this way is called an “interlayer jump” and, since there are three recording layers in the example shown in FIG. 20, other combinations also exist.
Here, if it is, for example, assumed that the respective base material thicknesses are 100 μm, 75 μm and 50 μm in a multilayer optical disc including three recording layers, coma aberrations created at the respective base material thicknesses are 100 mλ, 75 mλ and 50 mλ and largely differ when the optical disc is inclined by 1°. In the case of such a three-layer optical disc, if an objective lens is so moved as to focus a beam on a recording layer position as a destination of the interlayer jump with a tilt drive value immediately before the interlayer jump retained, i.e. with an optimal coma aberration correction amount maintained in the recording layer as a starting point of the interlayer jump when the above interlayer jump is performed, optimal coma aberration correction amounts differ among the respective recording layers as described above, wherefore the coma aberration of the recording layer as the destination of the interlayer jump cannot be corrected. As a result, a focus error signal is deteriorated, a focus lock-in operation in the recording layer as the destination of the movement of the focus position becomes unstable and a stable interlayer jump cannot be performed.